1. Field of the Invention
The present invention relates to a video signal processing circuit, a video signal processing method to be used in the video signal processing circuit, and an image display device using the same, which are used in, for example, a liquid crystal display device, plasma display device, or the like, and particularly to the video signal processing circuit being suitably used when there exists an invalid bit having no data corresponding to data making up an input video signal in a transmission format of an output video signal transmitted through a video signal line, and to a video signal processing method applicable to the video signal processing circuit.
2. Description of the Related Art
In a thin-type image display device such as a liquid crystal display device, plasma display device, or the like, as a display panel increases in size and comes to have high resolution (high definition) in recent years, a transmission frequency of a video signal in a device becomes higher. With increased size and high resolution of the display panel, unwanted EMI (Electro Magnetic Interference) emissions become high. Especially, a video signal clock and/or data signal contained in a video signal is transmitted at a comparably high speed and, therefore, EMI peak noises occur. A noise level limit of EMI emissions is regulated for every frequency and it is necessary that all noise levels are maintained within the regulated frequency band. Understandably, the smaller a noise level is, the better performance is. To take the best measure against EMI emissions, it is better that the number of occurring noises per unit time is made smaller.
Noises caused by a video signal clock and/or data signal vary depending upon a frequency component contained in a video signal to be transmitted. A plurality of frequency components contained in a transmission waveform causes an increase in the number of frequency spectra and an increase in the number of times of occurrence of peak noises per unit time causes a possibility that EMI emission levels exceed the regulated value. The frequency component of transmission waveform of a video signal should be minimized as much as possible and, by reducing the number of frequency spectrum being occurred, originally unwanted EMI spectrum is reduced and easy measure against EMI emissions is made possible. Since there is no clear rule for display pattern in assessing EMI emissions, it is desirable that EMI emission levels are low in any display pattern. For example, in the case when there exists an invalid bit having no data corresponding to data making up an input video signal in a transmission format, the number of ports (number of wirings) is necessarily increased due to a trend of high definition and/or an increase in size of a display panel, as a result, portions occupied by invalid bits increase and, therefore, when a polarity of the invalid bit is not controlled, the unwanted peak noise caused by a video signal becomes further large and, to take effective measures again EMI emissions, there is an increasing possibility that the number of shield members and/or number of parts increases. Therefore, an image display device is needed which can lower the EMI emission levels even when an invalid bit exists in a transmission format.
As related art of this type, an image display device disclosed in Patent Reference 1 (Japanese Patent Application Laid-open No. 2002-258802) is proposed. In the image display device, out of 6 bits making up each of necessary image signals MR, MG, and MB representing image data to be read from display memory, a bit other than bits to be used for display of an image with the number of gray level selected in a range less than the maximum number of gray levels are masked according to a gray-level control signal designating the number of the gray-level number. As a result, out of terminals of a display control circuit to output digital image signals OR, OG, and OB to be transmitted to a signal line driving circuit, an output terminal corresponding to the invalid bit, as shown in FIG. 12, is set to low level (“L”) or high level (“H”) in a fixed manner or is forced to a Hi-z (high impedance) state, in order to reduce power consumption by setting the terminal to “H” or “L” using an external resistance (pull-up resistance or pull-down resistance).
Moreover, a display device disclosed in Patent Reference 2 (Japanese Patent Application laid-open No. 2005-156786) has memory with a width of 32 bits which transmits data by assigning a polarity signal to a reminder of 8 bits being produced when video data containing 8 bits for each of R, G, and B data is stored. This eliminates necessity of increasing data lines simultaneously changing between a control device and memory, thereby reducing the EMI emissions.
Furthermore, a device to display an image disclosed in Patent Reference 3 (Japanese Patent Application Laid-open No. Hei op-2446572), when transmitting white and black gray-level data having a same value for each of the R, G, and B data, for example, transmits only data with R color and does not transmit data with other colors G and B. By forcing a signal line corresponding to G and B color data to a high impedance state, EMI emissions are reduced.
However, the above related technologies have the following problems. That is, in the image display device disclosed in the Patent Reference 1, if an invalid bit is set to “L” or “H” in a fixed manner, in some of gray-level data to be inputted, a change point (“H”→“L” or “L”→“H”) occurs in the transmission data. When the transmission data has the change point, a problem occurs that EMI emission noises occur in the corresponding period. For example, as shown in FIG. 13, in the image display device disclosed above, a transmission format of a video signal is transmitted via a mini-LVDS format of, for example, 10 bits, a video signal to be transmitted per one cycle is 10 bits×R (red), G (green), and B (blue) in total (that is, 30 bits) and, therefore, if data is transmitted through a video signal line in 4 pair 8 bit periods, there is a reminder of 2 bits. Ordinarily, a polarity (hereinafter, may be referred to as “logical level”) of the reminder bit (that is, invalid bit X1a, X2a, X1b and X2b) is set to “L” level (“0”) in a fixed manner to reduce power consumption. However, in the case of the method by which the logical levels of invalid bits X1a, X2a, X1b, and X2b are set in a fixed manner, when, in a predetermined period (that is, in a Data Cycle), a video signal having logical levels being reverse to the logical levels set in a fixed manner is inputted, many EMI peak noises caused by the video signal line occur.
When considered from a viewpoint of EMI emissions, as shown in, for example, in FIG. 14, when a signal having a gray level of 0 (“0000000000”) is inputted, since polarities of all bits are same, the EMI peak noise does not occur, however, as shown in FIG. 15, if a signal having a gray level of 1023 (“1111111111”) is inputted, since the invalid bits X1a, X2a, X1b, and X2b are set to “L” level in a fixed manner, a change in level occurs before and after the invalid bits X1a, X2a, X1b, and X2b. In this case, there is a problem that, when an image having a gray level of 1023, data 03 (±) and data 07 (±) changes in level in every 4 clock (CLK) period and, as a result, EMI peak noises caused by data occur in ¼ clock period.
In the display device disclosed in the Patent Reference 2, the polarity signal is used as a polarity inverted signal of a liquid crystal and there is no correlation between the polarity of the polarity inverted signal and input gray-level data. For example, if the level of all input gray level data is (“H”), polarity inversion for alternating-current driving of the liquid crystal is required and, if a change in level occurs from “H” to “L” transition at the time of polarity inversion, a change point occurs in transmission data, which presents another problem that EMI noise occurs during the period.
In the device disclosed in the Patent Reference 3, there is also a problem that, even when an invalid bit exists in the transmission format to transmit the above R data, no control is exerted, causing the occurrence of the EMI noises caused by data.